Organic light-emitting diodes (OLEDs), based on self-luminescence, enjoy the advantage of having a wide viewing angle and being able to be made thinner and lighter than liquid crystal displays (LCDs). In addition, an OLED display exhibits a very fast response time. Accordingly, OLEDs find applications in the illumination field as well as the full-color display field.
In general, the term “organic light-emitting phenomenon” refers to a phenomenon in which electrical energy is converted to light energy by means of an organic material. An OLED using the organic light-emitting phenomenon has a structure usually comprising an anode, a cathode, and an organic material layer interposed therebetween. In this regard, the organic material layer may be, for the most part, of a multilayer structure consisting of different materials, for example, a hole injecting layer, a hole transport layer, a light-emitting layer, an electron transport layer, and an electron injecting layer, in order to improve the efficiency and stability of the organic light-emitting diode (OLED). In the organic light-emitting diode having such a structure, when a voltage is applied between the two electrodes, a hole injected from the anode migrates to the organic layer while an electron is released from the cathode and moves toward the organic layer. In the luminescent zone, the hole and the electron recombine to produce an exciton. When the exciton returns to the ground state from the excited state, the molecule of the organic layer emits light. Such an organic light-emitting diode is known to have characteristics such as self-luminescence, high luminance, high efficiency, low driving voltage, a wide viewing angle, high contrast, and high-speed response.
Materials used as organic layers in OLEDs may be divided into luminescent materials and charge transport materials, for example, a hole injection material, a hole transport material, an electron injection material, and an electron transport material. As for the luminescent materials, there are two main families of OLED: those based on small molecules and those employing polymers. The light-emitting mechanism forms the basis for classification of the luminescent materials as fluorescent or phosphorescent materials, which use excitons in singlet and triplet states, respectively. Further, luminescent materials may be divided according to color into blue, green, and red light-emitting materials. Furthermore, yellow and reddish yellow light-emitting materials have been developed in order to achieve more natural colors.
Meanwhile, when a single material is employed as the luminescent material, intermolecular actions cause the wavelength of maximum luminescence to shift toward a longer wavelength, decreasing color purity or attenuating light with consequent reduction in the efficiency of the diode. In this regard, a host-dopant system may be used as a luminescent material so as to increase the color purity and the light emission efficiency through energy transfer.
This is based on the principle whereby, when a dopant is smaller in energy band gap than a host accounting for the light-emitting layer, the addition of a small amount of the dopant to the host generates excitons from the light-emitting layer so that the excitons are transported to the dopant, emitting light at high efficiency. Here, light of desired wavelengths can be obtained depending on the kind of dopant because the wavelength of the host moves to the wavelength range of the dopant.
With regard to related arts pertaining to host compounds in the light-emitting layer, reference may be made to Korean Patent No. 10-0910150 (Aug. 3, 2009), which discloses an OLED comprising a luminescent medium layer containing a compound in which an anthracene structure has a heterocyclic ring as a substituent at a terminal position thereof, and Japanese Patent No. 5608978 (Oct. 22, 2014), which describes on OLED comprising a luminescent medium layer containing an anthracene derivative in which an anthracene moiety has a dibenzofuran moiety as a substituent at a terminal position thereof.
Despite a variety of kinds of compounds prepared for use in luminescent media layers including the related art, there is still the continued need to develop organic layer materials capable of driving OLEDs at a lower voltage.